mmap_xen: Update to bitflags 2.4.0

[vireshk]

Clippy gives this warning currently:

error: &-masking with zero --> src/mmap_xen.rs:433:1	433	/ bitflags! { 434		/// Flags for the Xen mmap message. 435		pub struct MmapXenFlags: u32 { 436		/// Standard Unix memory mapping. ...	446		} 447		}		_^

= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bad_bit_mask
= note: `#[deny(clippy::bad_bit_mask)]` on by default
= note: this error originates in the macro `__impl_bitflags` which comes from the expansion of the macro `bitflags` (in Nightly builds, run with -Z macro-backtrace for more info)

Fix it.

Summary of the PR

Please summarize here why the changes in this PR are needed.

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• [ ] All added/changed functionality has a corresponding unit/integration test.
• [ ] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [ ] Any newly added unsafe code is properly documented.

[stefano-garzarella]

I don't know if it's a real bug or not, but if you want to keep the previous values, you can switch to bitflags 2.* and you shouldn't see the warning anymore.

[stefano-garzarella]

Not for this PR, but I was looking at the code, and I'm a bit suspicious of:

    /// Is standard Unix memory.
    pub fn is_unix(&self) -> bool {
        self.bits() == Self::UNIX.bits()
    }

Can a Unix memory have NO_ADVANCE_MAP flag set? If it is the case is_unix() will return false, so we may need a mask there.

[vireshk]

Not for this PR, but I was looking at the code, and I'm a bit suspicious of:

    /// Is standard Unix memory.
    pub fn is_unix(&self) -> bool {
        self.bits() == Self::UNIX.bits()
    }

Can a Unix memory have NO_ADVANCE_MAP flag set? If it is the case is_unix() will return false, so we may need a mask there.

Unix can't have NO_ADVANCE_MAP flag. Only Grant mapping can have that.

[stefano-garzarella]

Unix can't have NO_ADVANCE_MAP flag. Only Grant mapping can have that.

Oh I see, I was confused by the negative flag!